PATCH RFA: Support Plan 9 extensions in gcc

[Ian Lance Taylor]

"Joseph S. Myers" <joseph@codesourcery.com> writes:

> On Sun, 26 Sep 2010, Ian Lance Taylor wrote:
>
>> Joseph Myers asked specifically about standardization; I very much doubt
>> the Plan 9 folks care one way or another about whether this
>> functionality is added to the standard.  However, it seems to me
>> unlikely that any contradictory functionality would be added.
>
> It is also the case that we now know what form of anonymous structures and 
> unions is in C1X and I implemented support for that in GCC.

Thanks for the review.  You are correct that I didn't fully grasp the
fact that anonymous structures and unions are now implemented.

> However, one case of ambiguity that this patch does not detect appears to 
> conflict with the standard semantics.
>
> typedef struct { int a; } s1;
> struct s2 { s1; int s1; };
> int f(struct s2 *p) { return p->s1; }
>
> In C1X, p->s1 is an unambiguous reference to the int field.  With 
> -fplan9-extensions, there is ambiguity; this is not detected as such, but 
> instead you get an error:
>
> t1.c:3:23: error: incompatible types when returning type 's1' but 'int' was expected
>
> Logically this option should perhaps cause such a case to be detected with 
> an error for duplicate fields (and so be documented as causing some code 
> to be rejected that is otherwise accepted).

I have implemented this, and added a test case.

> The duplicate field errors for anonymous structures and unions post-date 
> your previous patch.  I don't actually see how the other cases of 
> ambiguity that your patch tries to detect can arise without an error being 
> given earlier for duplicate field names - they all involve multiple fields 
> with the same anonymous structure or union type, which would mean all the 
> fields within that type are duplicated.

I agree.  I have removed the ambiguity detection, and the code now
relies on the duplicate field test.

Here is the updated patch.  Bootstrapped and tested on
x86_64-unknown-linux-gnu.  OK for mainline?

Ian


gcc/ChangeLog:

2010-09-29  Ian Lance Taylor  <iant@google.com>

	* c-typeck.c (lookup_field): If -fplan9-extensions, permit
	referring to a field using a typedef name.
	(find_anonymous_field_with_type): New static function.
	(convert_to_anonymous_field): New static function.
	(convert_for_assignment): If -fplan9-extensions, permit converting
	pointer to struct to pointer to anonymous field.
	* c-decl.c (grokfield): If -fplan9-extensions, permit anonymous
	fields.
	(is_duplicate_field): New static function.
	(detect_field_duplicates_hash): If -fplan9-extensions, check for
	typedef names duplicating field names.
	(detect_field_duplicates): Likewise.
	* doc/invoke.texi (Option Summary): Mention -fplan9-extensions.
	(C Dialect Options): Document -fplan9-extensions.
	* doc/extend.texi (Unnamed Fields): Document -fplan9-extensions.

gcc/c-family/ChangeLog:

2010-09-29  Ian Lance Taylor  <iant@google.com>

	* c.opt (-fplan9-extensions): New option.

gcc/testsuite/ChangeLog:

2010-09-29  Ian Lance Taylor  <iant@google.com>

	* c-typeck.c (lookup_field): If -fplan9-extensions, permit
	referring to a field using a typedef name.
	(find_anonymous_field_with_type): New static function.
	(convert_to_anonymous_field): New static function.
	(convert_for_assignment): If -fplan9-extensions, permit converting
	pointer to struct to pointer to anonymous field.
	* c-decl.c (grokfield): If -fplan9-extensions, permit anonymous
	fields.
	(is_duplicate_field): New static function.
	(detect_field_duplicates_hash): If -fplan9-extensions, check for
	typedef names duplicating field names.
	(detect_field_duplicates): Likewise.
	* doc/invoke.texi (Option Summary): Mention -fplan9-extensions.
	(C Dialect Options): Document -fplan9-extensions.
	* doc/extend.texi (Unnamed Fields): Document -fplan9-extensions.


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